Electrochemical conversion of methane to higher hydrocarbon molecules, e.g., C2+ saturated hydrocarbons and C2+ unsaturated hydrocarbons is known. For example, the electrochemical conversion of methane has been accomplished in an electrocatalytic cell comprising an anode chamber, a cathode chamber, the anode chamber being separated from the cathode chamber by a membrane which prevents the flow of atoms, molecules, and electrons between the anode and cathode chambers, but permits the flow of ions. See, e.g., Chiang, et al., J. Eletrochem. Soc. 138, 6, L11, 12 (1991) and Hasakawa, et al., J. Electrochem. Soc. 140, 2, 459-462 (1993). In a conventional electrochemical cell, the anode chamber contains an anode in diffusive and electrical contact with the membrane, and the cathode chamber contains a cathode in diffusive and electrical contact with the membrane. The anode and cathode can be electrically connected via an external circuit, and the cell can be operated in either constant electric current or constant voltage mode.
In such a cell, methane may be conveyed to the anode chamber where the catalyst activates the methane to form CH3 fragments, a hydrogen ion (H+), and a free electron (e−). The hydrogen ions transit the membrane, and at the cathode produce a product comprising, e.g., molecular hydrogen in systems where oxygen is absent, utilizing the free electrons conducted from the anode to the cathode via the external circuit. In systems where oxygen is present, a product comprising water is produced at the cathode. In either case, the desired C2+ unsaturates are conducted away from the anode chamber.
In addition to activating the methane, the anode can further oxidize the CH3 fragments produced by the activation, resulting in the production of surface-bound CH2 fragments, which can in turn be oxidized to CH fragments. The CH fragments can be further oxidized to form carbon. Such carbon formation on the anode is the dominant reaction, resulting in a relatively small amount of C2+ hydrocarbon being produced by the cell.
Methods of increasing methane conversion to C2+ hydrocarbon have been described. For example, the use of an inorganic membrane in the cell enables the use of relatively high temperatures, thereby shifting equilibrium toward the formation of ethylene. The increase in ethylene, however, is accompanied by a loss in overall cell efficiency and an increase in carbon accumulation on the anode.
It is desired to increase the relative amount of C2+ hydrocarbon produced at the cell's anode, and in particular the relative amount of ethylene, while maintaining cell efficiency and lessening the amount of carbon accumulating on the cell's anode.